An antenna has been used since a long time ago as a device for converting a high-frequency current into an electromagnetic wave or converting an electromagnetic wave to a high-frequency current. An antenna is classified into a linear antenna, a planar antenna, a solid antenna, and the like according to its shape, or is classified into a dipole antenna, a monopole antenna, a loop antenna, and the like according to its structure. In particular, a loop antenna has a simple structure constituted by a single annular antenna element, and is one of antennas that are widely used to this date.
In response to diversified use of wireless communications, these antennas are expected to operate in various frequency bands. For example, a vehicle-mounted antenna is expected to operate in frequency bands for FM/AM broadcast, terrestrial digital broadcasting such as DAB (Digital Audio Broadcast), 3G (3rd Generation), LTE (Long Term Evolution), GPS (Global Positioning System), VICS® (Vehicle Information and Communication System), ETC (Electronic Toll Collection), and the like.
Conventionally, an antenna for operation in different frequency bands is often realized in the form of separate antenna devices which operate in the respective different frequency bands. For example, an antenna for FM/AM broadcast is provided as a whip antenna to be placed on a rooftop, and an antenna for terrestrial digital broadcasting is provided as a film antenna to be attached to a windshield.
However, an automobile has limited portions to which an antenna device can be attached. Further, an increase in the number of antenna devices to be attached results in problems such as spoiled design and increased attachment costs. To prevent such problems, it is effective to use an integrated antenna device. Note that the integrated antenna device refers to an antenna device which includes a plurality of antennas that operate in respective different frequency bands.
Examples of the integrated antenna device include ones described in Patent Literature 1 through 5. The integrated antenna device described in Patent Literature 1 includes an antenna for GPS and an antenna for ETC. The integrated antenna device described in Patent Literature 2 includes an antenna for 3G and an antenna for GPS. The integrated antenna described in Patent Literature 3 includes an antenna for ETC, an antenna for GPS, an antenna for VICS, a main antenna for telephone, and an auxiliary antenna for telephone. The integrated antenna device described in Patent Literature 4 includes an antenna for GPS, an antenna for ETC, an antenna for a first telephone, and an antenna for a second telephone. The integrated antenna device described in Patent Literature 5 includes an antenna that operates in a band of not lower than 100 kHz but not higher than 1 GHz (FM/AM broadcast, terrestrial digital broadcasting such as DAB, VICS, and the like), and an antenna that operates in a band of not lower than 1 GHz (GPS, satellite DAB, and the like).